1. Field of the Invention
The present invention relates to a device for creating print data utilized by a printer. The present invention further relates to a method for creating print data and a computer program product for creating print data. The printer of the present specification includes all devices for printing words or images onto a print medium. For example, the printer of the present specification includes ink jet printers and laser printers. Lic jet printers and laser printers include copying machines, fax machines, multifunctional products, etc.
2. Description of the Related Art
General printers may utilize colorant such as ink, toner, etc. to form points on a print medium. Desired words or images are thus formed on the print medium. In the present specification, a point formed on a print medium by a printer utilizing colorant is termed a dot. For example, in the case of an ink jet printer, a point formed on a print medium by discharging one droplet of ink from one nozzle toward the print medium is termed a dot. Furthermore, a point formed on a print medium by discharging a plurality of ink droplets onto the same location on the print medium from one or a plurality of nozzles is also termed a dot.
An ink jet printer discharges ink droplets from each nozzle towards a print medium, thus forming dots on the print medium. The ink jet printer may be utilized while connected with an external device such as a personal computer, or the like. In the case where an image shown on a display of the personal computer is to be printed by the ink jet printer, the personal computer creates bit-mapped data by dividing the image into a matrix shape. The bit-mapped data includes color information of coordinates disposed in the matrix shape. In the case of a color image, the color information of the bit-mapped data can be represented as a combination of color and color density of that color. Consequently, bit-mapped data of the color image includes a plurality of combinations of coordinate, color, and color density. Further, in the case of a monochrome image, the color information of the bit-mapped data can be represented as a combination of single color and color density, or can be represented only as color density. The bit-mapped data of the monochrome image includes a plurality of combinations of coordinate, single color, and color density (or a plurality of combinations of coordinate and color density).
The personal computer chooses coordinates at which dots will be formed from the bit-mapped data. This process is performed based on the color information of the coordinates included in the bit-mapped data. The coordinates are chosen from the bit-mapped data by using, for example, the halftone process or the dither method. The personal computer outputs the information representing the chosen coordinates to the ink jet printer. Here, this information is termed print data. The print data for color printing can be formed from, for example, a plurality of combinations of a chosen coordinate and color (for example, any one out of cyan, magenta, yellow, and black). The ink jet printer inputs the print data output from the personal computer, and forms dots based on the print data that has been input. For example, in the case where a combination of the coordinate (x, y) and yellow is included in the print data, the ink jet printer forms a yellow dot at the coordinate (x, y).
FIG. 12 is a figure schematically showing an example of dots 100 formed by the ink jet printer. In the example shown in FIG. 12, the dots 100 are formed in 8 rows×8 lines. In FIG. 12, the dots have not been filled with ink. The numbers shown above the dots 100 are row numbers, and the numbers shown to the left of the dots 100 are line numbers. Two adjacent dots 100 in the row direction or line direction overlap. The arrow A in the figure shows the direction of movement of the print medium with respect to the nozzles. Below, eight dots aligned in the A direction will be termed a dot row, and eight dots aligned in a direction orthogonal to the A direction will be termed a dot line. In this example, eight dot rows and eight dot lines are formed. Below, the rows will be represented as x coordinates and the lines will be represented as y coordinates. Sixty four coordinates (1, 1), (1, 2), etc. are included in the print data being utilized to form the sixty four dots 100 shown in this example.
Ink is discharged from the nozzles while the print medium is moving with respect to the nozzles in the direction of the arrow A. For example, the eight dots of the first dot row are formed by continuously discharging ink droplets from one nozzle while the print medium is moving with respect to the nozzle in the direction of the arrow A. Similarly, the eight dots of the second dot row are also formed by continuously discharging ink from one different nozzle. Eight nozzles are required to form the eight dot rows shown in FIG. 12. All the dots 100 are formed uniformly in the example shown in FIG. 12, and printing could be termed satisfactory.
However, it may happen that the eight nozzles for forming the eight dot rows are not formed in an evenly spaced manner in tee row direction. FIG. 13 shows an example of the dots 100 where the position of one of the nozzles in the row direction is slightly displaced. In this example, the nozzle for forming the fifth dot row is displaced towards the left. As a result, the fifth dot row is formed in a position that is displaced towards the left. In this case, there is a major overlap between the fourth dot row and the fifth dot row, and there is almost no overlap between the fifth dot row and the sixth dot row. The portion where there is a major overlap between the two adjacent dots is darker, and the portion where the two adjacent dots do not overlap is lighter. Consequently, it is considerably darker between the fourth dot row and the fifth dot row, and it is considerably lighter between the fifth dot row and the sixth dot row. The array pattern of the dots in the line direction (the A direction) is regular. As a result, dark portions and light portions are periodically repeated in the dark region between the fourth dot row and the fifth dot row. Further, dark portions and light portions are periodically repeated in the light region between the fifth dot row and the sixth dot row. It was understood from the research of the present inventors that the difference in density between the dark region and the light region is readily perceived when dark portions and light portions are periodically repeated in the dark region (or the light region). This perception of a difference in density between the dark region and the light region leads a user to feel that the printing result is bad.
The following technique is taught in Japanese Patent Application 7-323609. In this technique, an external device connected with an ink jet printer converts coordinates chosen from bit-mapped data. For example, in the case where a coordinate (1, 2) was chosen from the bit-mapped data, one coordinate is chosen randomly from four coordinates (hereafter referred to as sub-coordinates) located in the neighborhood of the coordinate (1, 2). The four sub-coordinates are aligned in the direction in which the print medium moves with respect to the nozzles. FIG. 14 (a) shows an example of the four sub-coordinates DO (1, 2), D1 (1, 2+t), D2 (1, 2+2 t), and D3 (1, 2+3 t). One sub-coordinate is chosen randomly from the four sub-coordinates D0 to D3. One coordinate is thus converted to one sub-coordinate. Information representing the chosen sub-coordinate is, included in print data. The external device outputs the print data to the ink jet printer. The ink jet printer forms dots based on the print data output from the external device. For example, in the case where the combination of the sub-coordinate D2 and black is included in the print data, a black dot is formed at the coordinate D2 (1, 2+2 t) on the print medium.
FIG. 14 (b) shows an example of twenty four dots formed by this technique. The sub coordinate where the dot is formed is a coordinate located in the neighborhood of the coordinate that was chosen from the bit-mapped data. Consequently, the dots can be formed on the print medium without there being any loss of the image information of the bit-mapped data. In this technique, the y coordinates of the sub-coordinates are chosen randomly, and as a result the dots are formed randomly in the y direction (the A direction). Consequently, even if there is displacement of the nozzles in the x direction, dark portions and light portions are not periodically repeated in the dark region (or the light region). As a result, the difference in density between the dark region and the light region is not readily perceived.
However, when the print data is formed by randomly choosing one sub-coordinate from four sub-coordinates, two adjacent dots may be mutually separated by a large distance. In this case, as shown by the symbol A′ in FIG. 14 (b), a large blank portion may be formed between two adjacent dots. Furthermore, even though a blank portion might not be formed, the quantity of overlap between the two adjacent dots may be extremely small. In the case where a dark region is formed in the neighborhood of the overlapping portion, this overlapping portion will appear comparatively lighter. In this case, this may appear as if a blank portion has been formed.
In the aforementioned technique, blank portions may be formed (or it may appear as if blank portions has been formed) even when a fully printed region was desired on the print medium. In this case, printing results are poor. In the aforementioned technique, the fin printing results may be poor.